In patent application PCT/GB94/00405 (Street), which is incorporated by reference herein, various embodiments of apparatus have been described through which two images may be observed, in the form of a stereoscopic image pair. A principal objective of that invention is to track the viewing position of the observer and to ensure that the light corresponding to each image correctly converges to the observer's respective eye locations. Certain embodiments employ one or two liquid crystal display (LCD) screens to achieve this objective. An embodiment, which uses a single LCD screen, relies on the vertical column structure of the LCD to match the pitch of a lenticular screen, placed immediately in front of the LCD surface. Two of the image columns behind each lenticular element provide a left and right eye view to the observer. A difficulty encountered with this approach is that most colour LCD's have a sub-pixel structure in which different colour elements are spaced horizontally. This can present a serious limitation to the use of standard LCD screens for autostereoscopic colour images.
A second embodiment overcomes this difficulty by employing two LCD's positioned at conjugate positions either side of a semitransparent mirror. Behind each LCD the combination of a striped mask or barrier strip screen and a matching lenticular, having the mask's pattern at the focal plane of the lenticular elements, provides a structured light source, the property of which is to pass light to defined regions at the position of the observer, whilst blocking light to adjacent regions. The system is configured to ensure that the light from one LCD is observed at locations spaced from those at which the light from the other LCD is observed. The lateral position and distance of these regions of concentration of light from the display apparatus is controlled to track the location of the observer so that each eye sees the correct image of the stereo pair, whereby a three-dimensional image is observed. The advantage of this arrangement is that the pattern of elements on the LCD need not be constrained by the geometry of the lenticular screen. Additionally, each LCD image retains its full complement of picture elements. There are occasions, however, when cost and/or space constraints make it attractive to employ a single LCD screen to provide both images for the stereo pair. The current invention achieves this whilst preserving the full observer tracking capability of the two LCD configuration described in PCT/GB94/00405. It does this by providing a specially adapted structured light source. This causes the light from alternate rows of elements within the array of picture elements comprising the LCD image to reach only one eye of the observer and the light from the other lines of picture elements only to reach the observer's other eye. In effect, two structured light sources similar to that described in PCT/GB94/00405 are spatially multiplexed to achieve the required result. Other embodiments of this invention provide a structured light source which can direct light from several groups of picture elements, arranged in lines or other configurations, to a corresponding plurality of different locations in space, making the simultaneous presentation of more than two perspectives of a scene possible.